1. Field of the Invention
The present invention relates to a method of manufacturing a device having a ferroelectric film, for example, a semiconductor device such as a nonvolatile memory device.
2. Description of the Prior Art
It is known that if an electric field is applied to a ferroelectric material such as PZT (lead(Pb) Zirconate Titanate), there occurs a state where the direction of polarization is aligned in the direction of the electric field, and this state remains after the electric field is removed. That is, the polarization of the ferroelectric material displays hysteresis relative to the application of the electric field. Consequently, it is possible to construct a nonvolatile semiconductor memory device by utilizing such hysteresis.
More specifically, a ferroelectric film is used as a gate insulation film of a field effect transistor. If an electric field is applied to the ferroelectric film to reverse the direction of polarization, the threshold value of the transistor varies between two values. Since the polarization of the ferroelectric film is held even after removing the electric field, information can be written/erased by applying the electric field to reverse the direction of polarization of the ferroelectric film. In addition, information can be read out by examining the threshold value of the transistor. In such a manner, information can be stored in a nonvolatile manner.
When a ferroelectric film is used in a semiconductor integrated circuit, the fine processing of the ferroelectric film is required. In patterning the ferroelectric film, the ferroelectric film is first formed on the surface of a semiconductor substrate, and a resist corresponding to a desired pattern is formed thereon. Then, the ferroelectric film is patterned by ion milling using the resist as a mask. However, in a ferroelectric material such as PZT which is the likeliest to be applied to a memory device, the selection ratio thereof to the resist in the ion milling is as low as 1 to 1.2. Therefore, it is necessary to apply the resist to large thicknesses of 2 to 3 .mu.m.
If the resist is applied thick, however, a resist pattern cannot be made fine, thereby to make it inevitably difficult to perform the fine processing of the ferroelectric film. Moreover, if even a part, which adheres to the surface of the semiconductor substrate, of the ferroelectric film is etched away by the ion milling, the semiconductor substrate is considerably damaged. Consequently, the characteristics of the device is degraded.
The prior art which is a solution to this problem is disclosed in, for example, a document entitled "Reactive ion beam etching of PLZT electrooptic substrates with repeated self-aligned masking" APPLIED OPTICS Vol. 25, No. 9, 1 May 1986, pp. 1508-1510. This document describes the technique for performing the fine processing of a PLZT (lead(Pb) Lanthanum Zirconate Titanate) substrate which is a ferroelectric substance. That is, a chrome thin film is formed on the surface of the PLZT substrate, and a thin resist pattern is formed on the chrome thin film. If dry etching is made using a resist as a mask by ion milling, the chrome thin film in a portion where there is no resist and the PLZT substrate directly thereunder are etched. At this time, the resist is also etched at approximately the same speed as PLZT. The reason for this is that the selection ratio of the resist to the PLZT is low as described above.
The dry etching is stopped in the stage in which etching proceeds to some extent, the resist remaining on the chrome thin film is removed and then, a resist is applied again. The resist is exposed from the back of the substrate (from the opposite side of the surface to which the resist is applied). At this time, the chrome thin film remaining on the surface of the substrate functions as a mask, so that the resist film can be patterned with high precision in a so-called self-alignment manner.
Thereafter, the same operations are repeated. Finally, the chrome thin film is removed, thereby to achieve the fine processing of the PLZT substrate.
However, fine processing performed by repeating the formation of the thin resist pattern and dry etching using the thin resist pattern as a mask is applicable to the processing of the PLZT substrate capable to transmitting light, but is not applicable to the fine processing of a ferroelectric film formed on, for example, the surface of a silicon substrate. That is, in the above-mentioned prior art, the transmittance for light of the PLZT is utilized, so that a thin resist pattern is formed in a self-alignment manner by exposure from the back of the substrate. Consequently, it is possible to form a fine resist pattern with high precision, thereby to make it possible to perform the fine processing of the PLZT substrate.
On the other hand, the exposure from the back of the substrate is not applicable to the processing of the ferroelectric film formed on the surface of the substrate having barrier properties for light such as a silicon substrate. Consequently, the same technique as described above cannot be used so as to perform the processing of the ferroelectric film. That is, even if an attempt to pattern the ferroelectric film by repeating the formation of the thin resist pattern and the dry etching is made, a resist pattern formed by a plurality of times of repetition is unavoidably shifted. Therefore, it is difficult to perform the fine processing of the ferroelectric film.
Moreover, even in the above-mentioned prior art, it is impossible to avoid the problem of damaging the substrate by the ion milling.